Nuclear magnetic resonance and theoretical studies in stereochemistry

Nuclear magnetic resonance (NMR) has become one of the most widely used techniques to provide structural information in analytical chemistry, physical chemistry and biochemistry. In NMR, individual nuclei in a molecule can be recognized by their chemical shifts, through-bond J couplings and through-space distances. Molecular structures and NMR properties of the molecules in interest can also be studied by theoretical quantum chemical computations. Ab initio and DFT calculations have been performed on a variety of amino acid and peptide hydrohalides. When generating the environment for the halide ion coordination, a simplified model is employed, using ammonium ions to replace –NH3+ and water to replace –OH groups. From the calculations of electric field gradient on halide ion site, the QCC and η are obtained and compared with the available experimental results of halogen NMR. Based on an approach analogous to COSY-based experiments in liquid-state NMR with coherences transferred via the weak scalar-coupling Hamiltonian, solid state double-quantum filtered correlation spectroscopy (DQF-COSY) was performed to study the 13C-enrichd thymidine. The results have shown the DQF-COSY method can also be applied to solid state with success. The effect of pseudorotation on carbon chemical shifts in tetrahydrofuran (THF) and 2,5-dimethyltetrahydrofuran (DMTHF) has been studied by theoretical quantum chemical calculations. Stable conformers along the pseudorotation pathway for THF and DMTHF were located using ab initio and DFT method under different basis sets. And the geometry and the chemical shift dependence on the pseudorotation angle for both molecules were analyzed. Hexamethylenetriperoxide diamine (HMTD), triacetone triperoxide (TATP) and diacetone diperoxide (DADP) are among the cyclic peroxide explosives most favored by terrorists. With solution NMR experiments in conjunction with a high level DFT calculations, two different conformers of the symmetry D3 and C2 were found to be coexist in solution for HMTD and TATP, their equilibrium constant was also determined. The studies showed the power of modern computational methods in predicting important physical parameters about these dangerous materials